Self-excited vibrations can occur in combustion chambers. These vibrations are due to a modulation of the quantity of gaseous fuel or fuel/air mixture injected into the combustion chamber through a nozzle, the modulation being caused by pressure fluctuations in the plane of the nozzle. The feedback circuit necessary for self-excitation is closed whenever the changes in the fuel consumption in the flame caused by the supply fluctuations satisfy a phase condition with the chamber pressure. A classical example of this is the so-called "singing flame".
Such vibrations can be dealt with, essentially, in two ways:
1. By modification of the acoustic properties, i.e., of the impedances of the fuel supply or the chamber. These measures, however, are only effective within a certain frequency band, as the impedances depend upon the frequency.
2. By acoustic decoupling of the fuel supply system through the use of an infinitely large inlet impedance. This impedance is attained by means of strong throttling of the fuel supply in the vicinity of the combustion chamber inlet, as provided for example, by sonic nozzles. This assumes that the fuel is supplied with a sufficiently high pressure, which is actually the case, or may be achieved in most cases. This measure acts independent of frequency; However, the application of this procedure does not make it possible to vary the fuel quantity over a wide range. A conventional control valve, which provides throttling and quantity adjustment, can only be applied outside of the total burner system in burners of conventional type. In this case, however, there still remains a supply system between the control valve and the combustion chamber inlet and, under certain circumstances, this supply system may participate in a vibration.
The objective of the invention is to produce a burner which combines in itself the decoupling, described above, of the fuel supply line in order to avoid combustion chamber vibrations with the possibility of controlling the quantity of fuel.
In accordance with the invention, this objective is achieved by means of a fuel lance of the type mentioned at the beginning having the characterising above.
The advantage of the invention is substantially to be seen in that a fuel lance is produced in a relatively simple manner, which fuel lance combines in itself in a compact manner both a decoupling of the fuel line in order to avoid combustion chamber vibrations and the possibility of fuel quantity control.
The decoupling is effective by virtue of the compact construction of the fuel lance, the distance between the throttle body and the nozzle outlet may be kept substantially shorter than the wavelength of typical natural vibrations of the combustion chamber system.
A further advantage of the invention is that the fuel lance may be provided with a central or radial nozzle outlet.
In the case of a supply of premixed fuel, an additional advantage of the present invention lies in the fact that the throttle body simultaneously acts as a flame trap.